Methods for preparing polyvinylidene fluoride composites

ABSTRACT

An electrically conductive composite comprising a polyvinylidene fluoride polymer or copolymer and carbon nanotubes is provided. Preferably, carbon nanotubes may be present in the range of about 0.5-20% by weight of the composite. 
     The composites are prepared by mixing or dispersing carbon nanotubes in polymer emulsion using an energy source such as a Waring blender. The liquid in the mixture is then evaporated to obtain the composite comprising the polymer and the nanotubes.

RELATED APPLICATIONS

This application is a continuation in part of U.S. Ser. No. 09/903,189,filed Jul. 11, 2001.

FIELD OF THE INVENTION

The invention relates generally to electrically conductivepolyvinylidene fluoride composites containing carbon nanotubes, and themethods for preparing them.

BACKGROUND OF THE INVENTION

Polyvinylidene Fluoride

Plastics are synthetic polymers which have a wide range of propertiesthat make them useful for a variety of applications ranging frompackaging and building/construction to transportation; consumer andinstitutional products; furniture and furnishings; adhesives, inks andcoatings and others. In general, plastics are valued for theirtoughness, durability, ease of fabrication into complex shapes and theirelectrical insulation qualities.

One such widely used plastic is polyvinylidene fluoride (—H₂C═CF₂—),(“PVDF”), which is the homopolymer of 1,1-difluoroethylene, and isavailable in molecular weights between 60,000 and 534,000. Thisstructure, which contains alternating —CH₂— and —CF₂— groups along thepolymer backbone, gives the PVDF material polarity that contributes toits unusual chemical and insulation properties.

PVDF is a semicrystalline engineered thermoplastic whose benefitsinclude chemical and thermal stability along with melt processibilityand selective solubility. PVDF offers low permeability to gases andliquids, low flame and smoke characteristics, abrasion resistance,weathering resistance, as well as resistance to creep and otherbeneficial characteristics. As a result of its attractive properties,PVDF is a common item of commerce and has a wide variety of applications(e.g., cable jacketing, insulation for wires and in chemical tanks andother equipments).

In addition to forming a homopolymer, PVDF also form co-polymers withother polymer and monomer families, most commonly with the co-monomershexafluoropropylene (HFP), chlorotrifluoroethylene (CTFE), andtetrafluoroethylene (TFE), as well as terpolymers and olefins. Theproperties of the copolymers is strongly dependent on the type andfraction of the co-monomers as well as the method of polymerization. Forexample, HFP makes a homogenous copolymer with PVDF. On the other hand,the PVDF copolymer phase segregates if the other monomer is notfluorinated.

Conductive Plastics

Recently, demand and applications for electrically conductive plasticshave grown. In these uses, one seeks to exploit the unique properties ofplastics, often as an alternative to metals. For example, electricallyconductive polymeric materials are desirable for many applicationsincluding the dissipation of electrostatic charge from electrical parts,electrostatic spray painting and the shielding of electrical componentsto prevent transmission of electromagnetic waves.

Conductivity (i.e., the ability of material to conduct or transmit heator electricity) in plastics is typically measured in terms of bulkresistivity (i.e., volume resistivity). Bulk resistivity, which is theinverse of conductivity, is defined as the electrical resistance perunit length of a substance with uniform cross section as measured inohm-cm. Thus, in this manner, the electrical conductivity of a substanceis determined by measuring the electrical resistance of the substance.

Electrically conductive plastics can be divided into several categoriesaccording to their use. For example, high level of resistivity (i.e.,low level of conductivity) ranging from approximately 10⁴ to 10⁸ ohm/cmgenerally confer protection against electrostatic discharge (“ESD”) andis referred to as the ESD shielding level of conductivity. This is alsothe level of conductivity needed for electrostatic painting. The nextlevel of resistivity, which ranges from approximately 10⁴ ohm/cm andlower, protects components contained within such plastic againstelectromagnetic interference (“EMI”) as well as prevents the emission ofinterfering radiation, and is referred to as the EMI shielding level ofconductivity. In order for a plastic article to be used as a conductiveelement like a current collector or separator plate in anelectrochemical cell, resistivity less than 10² ohm/cm is required.

The primary method of increasing the electrical conductivity of plasticshave been to fill them with conductive additives such as metallicpowders, metallic fibers, ionic conductive polymers, intrinsicallyconductive polymeric powder, e.g., polypyrrole, carbon fibers or carbonblack. However, each of these approaches has some shortcomings. Metallicfiber and powder enhanced plastics have poor corrosion resistance andinsufficient mechanical strength. Further, their density makes highweight loadings necessary. Thus, their use is frequently impractical.

When polyacrylonitrile (“PAN”) or pitch-based carbon fiber is added tocreate conductive polymers, the high filler content necessary to achieveconductivity results in the deterioration of the characteristicsspecific to the original resin. If a final product with a complicatedshape is formed by injection molding, uneven filler distribution andfiber orientation tends to occur due to the relatively large size of thefibers, which results in non-uniform electrical conductivity.

Principally because of these factors and cost, carbon black has becomethe additive of choice for many applications. The use of carbon black,however, also has a number of significant drawbacks. First, thequantities of carbon black needed to achieve electrical conductivity inthe polymer or plastic are relatively high, i.e. 10-60%. Theserelatively high loadings lead to degradation in the mechanicalproperties of the polymers. Specifically, low temperature impactresistance (i.e., a measure of toughness) is often compromised,especially in thermoplastics. Barrier properties also suffer. Sloughingof carbon from the surface of the materials is often experienced. Thisis particularly undesirable in many electronic applications. Similarly,outgassing during heating may be observed. This adversely affects thesurface finish. Even in the absence of outgassing, high loadings ofcarbon black may render the surface of conductive plastic partsunsuitable for automotive use.

Taken as a whole, these drawbacks limit carbon black filled conductivepolymers to the low end of the conductivity spectrum. For EMI shieldingor higher levels of conductivity, the designer generally resorts tometallic fillers with all their attendant shortcomings or to metalconstruction or even machined graphite.

What ultimately limits the amount of carbon black that can be put intoplastic is the ability to form the part for which the plastic is desiredfor. Depending on the plastic, the carbon black, and the specific partfor which the plastic is being made, it becomes impossible to form aplastic article with 20-60 wt % carbon black, even if the physicalproperties are not critical.

Carbon Fibrils

Carbon fibrils have been used in place of carbon black in a number ofpolymer applications. Carbon fibrils, referred to alternatively asnanotubes, whiskers, buckytubes, etc., are vermicular carbon depositshaving diameters less than 1.0μ and usually less than 0.2μ. They existin a variety of forms and have been prepared through the catalyticdecomposition of various carbon-containing gases at metal surfaces. Suchfibers provide significant surface area when incorporated into astructure because of their size and shape. They can be made with highpurity and uniformity.

It has been recognized that the addition of carbon fibrils to polymersin quantities less than that of carbon black can be used to produceconductive end products. For example, U.S. Pat. No. 5,445,327, herebyincorporated by reference, to Creehan disclosed a process for preparingcomposites by introducing matrix material, such as thermoplastic resins,and one or more fillers, such as carbon fibers or carbon fibrils, into astirred ball mill. Additionally, U.S. Ser. No. 08/420,330, entitled“Fibril-Filled Elastomer Compositions,” also incorporated by reference,disclosed composites comprising carbon fibrils and an elastomericmatrix, and methods of preparing such.

It has also been recognized that the addition of carbon fibrils topolymers can be used to enhance the tensile and flexural characteristicsof end products. (See, e.g. Goto et al., U.S. application Ser. No.511,780, filed Apr. 18, 1990, and hereby incorporated by reference.)

Additionally, prior work by Moy et al., U.S. application Ser. No.855,122, filed Mar. 18, 1992, and Uehara et al., U.S. application Ser.No. 654,507, filed Feb. 23, 1991, both incorporated by reference,disclosed the production of fibril aggregates and their usage increating conductive polymers. Moy et al. disclosed the production of aspecific type of carbon fibril aggregate, i.e. combed yarn, and alludedto its use in composites. Uehara et al. also disclosed the use of fibrilaggregates in polymeric materials. The fibril aggregates have apreferred diameter range of 100-250 microns. When these fibrilaggregates are added to polymeric compositions and processed,conductivity is achieved.

U.S. Pat. No. 5,643,502 to Nahass et al., hereby incorporated byreference, disclosed that a polymeric composition comprising a polymericbinder and 0.25-50 weight % carbon fibrils had significantly increasedIZOD notched impact strength (i.e., greater than about 2 ft-lbs./in) anddecreased volume resistivity (i.e., less than about 1×10¹¹ ohm-cm).Nahass disclosed a long list of polymers (including polyvinylidenefluoride) into which carbon fibrils may be dispersed to form acomposite. The polymers used by Nahass in the Examples of the '502patent for preparing conductive, high toughness polymeric compositionsinclude polyamide, polycarbonate, acrylonitrile-butadiene-styrene, poly(phenylene ether), and thermoplastic urethane resins and blends.

While the nanotube-containing polymer composites of the art are usefuland have valuable strength and conductivity properties, many new usesfor such composites require that very high strength and low conductivitybe achieved with low nanotube loading in the polymer. Accordingly, theart has sought new composite compositions which achieve these ends.

OBJECTS OF THE INVENTION

It is a primary object of the invention to provide a polymer compositewhich is mechanically strong and electrically conductive.

It is a particular object of the invention to provide a polymercomposite which has a higher level of conductivity than known polymercomposites.

It is yet another object of the invention to provide polymer compositeswhich achieve extraordinary levels of conductivity at low levels ofnanotube loading.

It is a further object of the invention to provide methods for preparinga polymer composite which is mechanically strong and electricallyconductive.

SUMMARY OF THE INVENTION

It has now been discovered that composites containing polyvinylidenefluoride polymer or copolymer and carbon nanotubes have extraordinaryelectrical conductivity. Composites with less than 1% by weight ofcarbon nanotubes have been found to have a bulk resistivity many timeslower than the bulk resistivity of other polymer composites havingsimilar nanotube loading. Composites with as little as 13% by weightcarbon nanotubes have a bulk resistivity similar to that of pure carbonnanotube mats.

Composites containing polyvinylidene fluoride polymer or copolymer andcarbon nanotubes may be prepared by dissolving the polymer in a solventto form a polymer solution and then adding the carbon nanotubes into thesolution. The solution is mixed using a sonicator or a Waring blender. Aprecipitating component is added to precipitate out a compositecomprising the polymer and the nanotubes. The composite is isolated byfiltering the solution and drying the composite.

Composites containing polyvinylidene fluoride polymer or copolymer andcarbon nanotubes may also be prepared by adding carbon nanotubes to apolymer emulsion and then mixing the emulsion with a Waring Blender. Thewater/solvent is then removed by evaporation to obtain the compositecomprising the polymer and the nanotubes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph plotting composite resistivity as a function ofnanotube loading in a PVDF/HFP composite.

FIG. 2 is a graph plotting composite resistivity as a function ofgraphite concentration for PVDF composites with 13% nanotube loading.

FIG. 3 is a logarithmic graph plotting resistivity as a function ofnanotube weight fraction for various polymer composites.

FIG. 4 is a graph plotting composite conductivity as a function ofnanotube loading in various polymer composites.

DETAILED DESCRIPTION OF THE INVENTION

PVDF-Nanotube Composites

It has now been discovered that composites containing PVDF polymer orcopolymer and carbon nanotubes have electrical conductivities muchhigher than other polymer/carbon nanotube composites known in the art.As used hereafter, the term “PVDF composite” refers broadly to anycomposite containing PVDF or a copolymer of vinylidene fluoride andanother monomer, and carbon nanotubes. Unlike other polymer composites,PVDF composites with bulk resistivities as low as pure carbon nanotubescan be formed. For simplicity, the term percent nanotube loading (%nanotube loading) will be used to refer to percentage of nanotube byweight in the composite.

It has now been discovered that lower loadings of nanotubes in PVDFcomposites results in far higher conductivities than similar loadings inother polymer composites. For example, a PVDF composite with 5% nanotubeloading has a bulk resistivity of 0.42 ohm-cm while a poly(paraphenylenesulfide) composite with 5% nanotube loading has a bulk resistivity of3.12 ohm-cm.

PVDF composites containing carbon nanotubes in an amount as little as 1%or less by weight have an exceptionally low bulk resistivity compared tothe pure PVDF polymer or copolymer, and have exceptionally lowresistivity compared to other polymer composites at similar nanotubeloadings. Nanotube loading may be widely varied. For example, PVDFcomposites may be made with nanotube loadings of broadly from 0.01-30%desirably from 0.5-20% and preferably from 1-15%. It has been found thatPVDF composites have much lower bulk resistivities compared to otherpolymer composites at any given nanotube loading.

It has been further discovered that a PVDF composite with as little asabout 13% nanotube loading has a bulk resistivity comparable to a purenanotube mat, i.e., between 0.02 ohm-cm to 0.08 ohm-cm. PVDF compositeswith about 13% to 20% nanotube loading all have bulk resistivity valueswithin the range of the resistivity values of a pure nanotube mat. PVDFcomposites can be formed with bulk resistivity of less than about 10ohm-cm or less than about 1 ohm-cm. The bulk resistivity of PVDFcomposite may be adjusted by varying the nanotube loading to meet thelevel of conductivity required for its intended application.

Depending on how the composite is prepared, no further improvements inconductivity beyond 13-20% nanotube loading in PVDF were observed, thelimiting resistivity of 0.02 ohm-cm (i.e., the resistivity of a pure matof carbon nanotubes) having been reached. The lower limit of nanotubeloading is set by the limit of percolation and will depend on variousfactors such as the method of composite formation, the materials used,etc. For example, Table 1 shows that the lower limit of nanotube loadingunder the conditions in Example 1 is well under 1%, but apparently above0.2%.

The monomers which may be used with vinylidene fluoride monomer to formPVDF copolymers for the composites of the invention includehexafluoropropylene, polystyrene, polypropylene, CTFE, TFE, terpolymersor olefins. The copolymers may be produced broadly from a de minimsamount of a monomer other than vinylidene fluoride to as much as 90% byweight of such monomer. Desirably copolymers of the invention containfrom 1% to 70% by weight of such other monomer and preferably from 10%to 50% by weight thereof.

The PVDF composites of the invention also include mixtures of PVDF andother polymers, including those wherein the PVDF and other polymers aremiscible or immiscible with one another. The PVDF composites of theinvention also include mixtures of PVDF and copolymers formed fromvinylidene fluoride and another monomer, as described above, andmixtures of these mixtures with other polymers.

Fillers such as graphite may also be used with PVDF copolymercomposites.

Carbon Nanotubes

A variety of different carbon nanotubes may be combined with PVDF orPVDF copolymers to form the composites of the present invention.Preferably, the nanotubes used in the invention have a diameter lessthan 0.1 and preferably less than 0.05 micron.

U.S. Pat. No. 4,663,230 to Tennent, hereby incorporated by reference,describes carbon fibrils that are free of a continuous thermal carbonovercoat and have multiple ordered graphitic outer layers that aresubstantially parallel to the fibril axis. U.S. Pat. No. 5,171,560 toTennent et al., hereby incorporated by reference, describes carbonnanotubes free of thermal overcoat and having graphitic layerssubstantially parallel to the fibril axes such that the projection ofsaid layers on said fibril axes extends for a distance of at least twofibril diameters. As such, these Tennent fibrin may be characterized ashaving their c-axes, the axes which are perpendicular to the tangents ofthe curved layers of graphite, substantially perpendicular to theircylindrical axes. They generally have diameters no greater than 0.1μ andlength to diameter ratios of at least 5. Desirably they aresubstantially free of a continuous thermal carbon overcoat, i.e.,pyrolytically deposited carbon resulting from thermal cracking of thegas feed used to prepare them. These fibrils are useful in the presentinvention. These Tennent inventions provided access to smaller diameterfibrils having an ordered outer region of catalytically grown multiple,substantially continuous layers of ordered carbon atoms having anoutside diameter between about 3.5 to 70 nm, and a distinct inner coreregion, each of the layers and the core being disposed substantiallyconcentrically about the cylindrical axis of the fibrils, said fibrilsbeing substantially free of pyrolytically deposited thermal carbon.Fibrillar carbons of less perfect structure, but also without apyrolytic carbon outer layer have also been grown.

Geus, U.S. Pat. No. 4,855,091, hereby incorporated by reference,provides a procedure for preparation of fishbone fibrils substantiallyfree of a pyrolytic overcoat. When the projection of the graphiticlayers on the nanotube axis extends for a distance of less than twonanotube diameters, the carbon planes of the graphitic nanotube, incross section, take on a herring bone appearance. Hence, the termfishbone fibrils. These carbon nanotubes are also useful in the practiceof the invention.

The “unbonded” precursor nanotubes may be in the form of discretenanotubes, aggregates of nanotubes, or both.

Nanotubes aggregate in several stages or degrees. Catalytically grownnanotubes produced according to U.S. Ser. No. 08/856,657, filed May 15,1997, hereby incorporated by reference, are formed in aggregatessubstantially all of which will pass through a 700 micron sieve. About50% by weight of the aggregates pass through a 300 micron sieve. Thesize of as-made aggregates can, of course, be reduced by various means,but such disaggregation becomes increasingly difficult as the aggregatesget smaller.

Nanotubes may also be prepared as aggregates having various morphologies(as determined by scanning electron microscopy) in which they arerandomly entangled with each other to form entangled balls of nanotubesresembling bird nests (“BN”); or as aggregates consisting of bundles ofstraight to slightly bent or kinked carbon nanotubes havingsubstantially the same relative orientation, and having the appearanceof combed yarn (“CY”) e.g., the longitudinal axis of each nanotube(despite individual bends or kinks) extends in the same direction asthat of the surrounding nanotubes in the bundles; or, as aggregatesconsisting of straight to slightly bent or kinked nanotubes which areloosely entangled with each other to form an “open net” (“ON”)structure. In open net structures, the extent of nanotube entanglementis greater than observed in the combed yarn aggregates (in which theindividual nanotubes have substantially the same relative orientation)but less than that of bird nest. CY and ON aggregates are more readilydispersed than BN making them useful in composite fabrication whereuniform properties throughout the structure are desired.

The morphology of the aggregate is controlled by the choice of catalystsupport. Spherical supports grow nanotubes in all directions leading tothe formation of bird nest aggregates. Combed yarn and open nestaggregates are prepared using supports having one or more readilycleavable planar surfaces, e.g., an iron or iron-containing metalcatalyst particle deposited on a support material having one or morereadily cleavable surfaces and a surface area of at least 1 squaremeters per gram. Moy et al., U.S. application Ser. No. 08/469,430entitled “Improved Methods and Catalysts for the Manufacture of CarbonFibrils”, filed Jun. 6, 1995, hereby incorporated by reference,describes nanotubes prepared as aggregates having various morphologies.

Further details regarding the formation of carbon nanotube or nanofiberaggregates may be found in U.S. Pat. No. 5,165,909 to Tennent; U.S. Pat.No. 5,456,897 to Moy et al.; Snyder et al., U.S. patent application Ser.No. 149,573, filed Jan. 28, 1988, and PCT Application No. US89/00322,filed Jan. 28, 1989 (“Carbon Fibrils”) WO 89/07163, and Moy et al., U.S.patent application Ser. No. 413,837 filed Sep. 28, 1989 and PCTApplication No. US90/05498, filed Sep. 27, 1990 (“Fibril Aggregates andMethod of Making Same”) WO 91/05089, and U.S. application Ser. No.08/479,864 to Mandeville et al., filed Jun. 7, 1995 and U.S. applicationSer. No. 08/329,774 by Bening et al., filed Oct. 27, 1984 and U.S.application Ser. No. 08/284,917, filed Aug. 2, 1994 and U.S. applicationSer. No. 07/320,564, filed Oct. 11, 1994 by Moy et al., all of which areincorporated by reference.

Other fibrils of different microscopic and macroscopic morphologiesuseful in the present invention include the multiwalled fibrilsdisclosed in U.S. Pat. Nos. 5,550,200, 5,578,543, 5,589,152, 5,650,370,5,691,054, 5,707,916, 5,726,116, and 5,877,110 each of which areincorporated by reference.

Single walled fibrils may also be used in the composites of theinvention. Single walled fibrils and methods for making them aredescribed in U.S. Pat. No. 6,221,330 and WO 00/26138, both of which arehereby incorporated by reference. Single walled fibrils havecharacteristics similar to or better than the multi-walled fibrilsdescribed above, except that they only have a single graphitic outerlayer, the layer being substantially parallel to the fibril axis.

PVDF composites containing carbon nanotubes with different grades,sizes, morphologies, or types have different bulk resistivity at a givenfibrinl loading. For example, it has been found that combed candy (“CC”)nanotubes provide lower bulk resistivity than bird nest (“BN”) nanotubesin PVDF composites at low nanotube loading. Without wishing to be boundby any theory, it is believed that CC nanotubes, which are aggregated inparallel bundles, are easier to disperse in the polymer than BNnanotubes, resulting in a more even distribution of fibrils in thecomposite and hence, lower bulk resistivity.

The conductivity levels obtained using PVDF composites formed from PVDFpolymer or copolymers and carbon nanotubes make it possible to useconductive plastic, with all its property and fabrication advantages, inplace of metals or pure graphite in a number of applications.

Use of PVDF Composites

PVDF composites of the invention may be used in applications whereexceptional electrical conductivity is important. Examples of such usesinclude current collectors for high power electrochemical capacitors andbatteries. Current commercial materials used for these purposes havebulk resistivities of approximately 1 ohm-cm. Other applications includeconducting gaskets or EMF shield coatings. In these applications, adifference of, for example, 0.04 ohm-cm in bulk resistivity will have avery significant impact on product performance.

Still further uses include bipolar plates for PEM fuel cells as well asbifunctional (binder and conductivity enhancers) additives to a lithiumbattery cathode. These bipolar plates are formed by preparing a PVDFcomposite as disclosed herein and then extruding a PVDF composite sheetwith a thickness of, for example, 2 mm. A single screw extruder may beused for the sheet extrusion. Flow channels may be engraved between twohot plates, one with a mirror pattern of the front plate channel and theother with a mirror pattern of the back plate channel. The channels mayrun parallel to each other from one corner to another, with each channelseparated from the other by 0.5 mm. The channels may have a width anddepth of 0.5 mm.

Method of Preparing Composites

PVDF composites may be prepared by a solution method in which PVDFpolymer or copolymer is dissolved in a solvent such as acetone to form asolution. Other soluble solvents such as tetrahydrofuran, methyl ethylketone, dimethyl formamide, dimethyl acetamide, tetramethyl urea,dimethyl sulfoxide, trimethyl phosphate, 2-pyrrolidone, butyrolacetone,isophorone, and carbitor acetate may be used.

Nanotubes are dispersed in the solvent by applying energy to thepolymer-nanotube mixture. The energy source can be a mechanicalhomogenizer, ultrasonic sonifier, high speed mixer, Waring blender, orany other mixing means known in the art. A precipitating component suchas water is added to precipitate or quench the solid compositecontaining the polymer and the nanotubes. The precipitating componentmay be any medium which is miscible with the solvent, but in which thePVDF polymer or copolymer mixture is insoluble.

The solvent may optionally be removed by filtration or evaporation anddried to isolate the PVDF composite. The composite may be isolated bydrying or evaporating steps such as heat drying, vacuum drying,freeze-drying, etc. known in the art.

PVDF composites may also be prepared by a melt compounding process inwhich the PVDF polymer or copolymer is mixed with nanotubes in themixing head of a mixer such as a Brabender mixture at high temperatures(i.e., over 200° C.) to melt and compound the PVDF polymer or copolymerinto the carbon nanotubes to form the composite.

Once the composite has been obtained, it may then be molded as necessaryusing compression or injection molding equipment and methods known inthe art.

PVDF composites prepared using the solvent solution method havesignificantly lower bulk resistivity and thus were better electricalconductors, than PVDF composites made using traditional melt compoundingmethods. Without wishing to be bound by any theory, it is believed thatthe solvent solution method allows for better intermixing of the PVDFwith the carbon nanotubes in the PVDF composites, thus resulting inlower bulk resistivity.

It has also been found that at low nanotube loadings, using sonicatorsor ultrasonic sonifiers resulted in PVDF composites having lower bulkresistivities than PVDF composites made using mechanical mixing meanssuch as Waring blenders. Without wishing to be bound by any theory, itis also believed that sonicators or ultrasonic sonifiers are able tobetter disperse low levels of nanotubes in the polymer than mechanicalmixing means, resulting in better distribution of nanotubes within thecomposite and hence, better conductivity.

For any industrial processes where use of organic solvents is notpreferred, PVDF composites may be prepared using PVDF emulsions. In thismethod, carbon nanotubes are directly mixed with or dispersed in PVDFemulsions (or PVDF latex, or any dispersions of PVDF in water) byapplying an energy source such as a mechanical homogenizer, ultrasonicsonifier, high speed mixer, Waring blender, or any other mixing meansknown in the art. The water (or liquid) in the mixture is then removed,for example, by evaporation or any drying means known in the art torecover the PVDF composite.

EXAMPLES

Examples of electrically conductive PVDF composites and methods ofpreparing the same are set forth below.

Example I

A PVDF polymer, Kynar 761, was obtained from Elf Atochem and dissolvedin acetone. Hyperion CC carbon nanotubes were added and dispersed intothe polymer solution for two to five minutes using a high shear blender(i.e., Waring blender). Water was added to the dispersion to precipitateout the polymer with the nanotubes. The material was filtered and thefiltrate was dried in a vacuum oven at 100° C. to remove acetone andwater, leaving behind the dry nanotube/PVDF composite. Multiple sheetsof the composite with thickness of 0.003-0.01 inches were made using acompression molder. Bulk resistivity of the thin sheet samples wasmeasured using a four probe method. Tensile strength was also measured.Multiple batches with different amounts of carbon nanotubes were tested.The results are reported in Table 1 below:

TABLE 1 Nano- Nano- Thick- Tensile tubes PVDF tubes ness StrengthResistivity Batch # (g) (g) (%) (inch) (psi) (Ohm-cm) 1 .02 10 0.20 — —300,000 2 .09 10 0.89 0.0075 7005 73.7404 0.0107 7005 64.5745 0.00357005 88.5977 0.0109 7005 87.3533 3 .11 10 1.09 0.0102 8145 22.55720.0058 8145 15.7831 4 .31 10 3.01 0.0090 8072 1.2106 0.0099 8072 1.21890.0075 8072 1.3074 0.0067 8072 1.2219 5 .53 10 5.03 0.0055 6739 0.39240.0080 6739 0.6255 0.0100 6739 0.4890 0.0050 6739 0.3912 6 .61 8 7.080.0030 6770 0.2934 0.0040 6770 0.2992 0.0030 6770 0.2554 0.0098 67700.2819 7 1.5 15 9.09 0.0078 8025 0.2154 0.0100 8025 0.2359 0.0081 80250.2470 0.0090 8025 0.2175 8 1.24 10 11.03 0.0115 7144 0.1336 0.0132 71440.1177 0.0142 7144 0.1152 0.0130 7144 0.1496 9 1.2 8 13.04 0.0115 75210.1012 0.0125 7521 0.0811 0.0108 7521 0.0938 0.0081 7521 0.0769 10 1.5 620.00 0.0120 5318 0.0387 0.0110 5318 0.0443 0.0065 5318 0.0441 0.00755318 0.0381 0.0130 5318 0.0419 11 2 6 25.00 0.0097 1918 0.0391 0.00901918 0.0371 0.0090 1918 0.0497 0.0120 1918 0.0483

The results of Example I show that a PVDF composite with less than 1%nanotube loading had a significantly lower bulk resistivity than a purePVDF polymer. The bulk resistivity of the composites droppedsignificantly as the nanotube loading increased to approximately 3%. Atapproximately 5% nanotube loading, the bulk resistivity of the PVDFcomposite was below 1 ohm-cm. At approximately 13% nanotube loading, thebulk resistivity approached 0.08 ohm-cm, which is comparable to that ofa pure CC nanotube mat. At nanotube loadings higher than 13% (i.e.,13-25%) the PVDF composite had bulk resistivities within the ranges ofthose of pure CC nanotube mats.

Example II

Using the procedure of Example I and Kynar 761 PVDF, a comparison wasmade of the resistivity of composites prepared with Hyperion CCnanotubes and with Hyperion BN nanotubes, respectively. The results arereported in Table 2 below:

TABLE 2 CC Nanotubes BN Nanotubes Tensile Tensile Nanotubes ResistivityStrength Resistivity Strength Batch # (%) (ohm-cm) (psi) (ohm-cm) (psi)1 1 19.17 8145 12987 — 2 5 0.4242 6739 4.1081 6663 3 7 0.2825 67701.6136 7227 4 11 0.1290 7144 0.4664 7201

The results of Example II confirm that at low nanotube loadings, PVDF/CCnanotube composites have significantly lower bulk resistivity thanPVDF/BN nanotube composites.

Example III

Example I was repeated, except that an ultrasound sonicator or ahomogenizer was used instead of a Waring blender to disperse thenanotubes in the polymer solution. The results are reported below inTable 3:

TABLE 3 Nanotubes Thickness Dispersion Resistivity Batch # (%) (inch)Method (ohm-cm) 1 1.05 0.0058 Sonicator 11.7122 2 3.03 0.0080 Sonicator0.6144 3 13.04 0.0130 Sonicator 0.0924 0.0110 Sonicator 0.0959 4 13.040.0100 Homogenizer 0.1168 0.0110 Homogenizer 0.1028 5 20.00 0.0090Homogenizer 0.0381 0.0090 Homogenizer 0.0509

These results revealed that PVDF composites with low carbon nanotubeloadings made with a sonicator generally had lower bulk resistivitiesthan composites which were made with a Waring blender. PVDF compositesmade with a homogenizer had similar bulk resistivity values to thosemade with a Waring blender.

Example IV

Example I was repeated, except that the nanotubes were heat treatedunder hydrogen, argon, or air before they were dispersed into thepolymer solution. Heat treatment of the nanotubes was carried out byheating the nanotubes under a flowing gas at the following conditions:hydrogen—600° C. for 30 minutes; argon—1000° C. for 30 minutes. Airoxidation was carried out by heating the nanotubes in an oven in air at450° C. for 2 hours. The results are reported below in Table 4:

TABLE 4 Original Nanotubes from H₂ Treated Ar Treated Air OxidizedExample1 Nanotubes Nanotubes Nanotubes Tensile Tensile Tensile R TensileR Strength R Strength R Strength (ohm- Strength Fibril % (ohm-cm) (psi)(ohm-cm) (psi) (ohm-cm) (psi) cm) (psi) 1.09 19.1701 8145 11.6651 7456167.0608 7404 609.9 6995 5.03 0.4242 6739 0.4751 6578 0.7111 6648 2.67407474 20.00 0.0422 5318 0.0514 5456 0.0783 7855 0.1942 7051 25.00 0.04571919 0.0327 2721 — — — —

Generally, heat treatment of nanotubes under hydrogen or argon did notimprove the conductivity of the PVDF composites as compared tocomposites formed from nontreated nanotubes. PVDF composites formed fromair oxidized nanotubes showed significantly poorer conductivity (i.e.,higher bulk resistivity) but higher tensile strength (at 5-20% fibrilloading) compared to composites formed from nontreated nanotubes.

Example V

Polyvinylidene fluoride-hexafluoropropylene (i.e., PVDF/HFP) copolymerwas obtained from Solvay Advanced Polymers (21508) and the procedure ofExample I was repeated with the PVDF/HFP copolymer instead of the purePVDF Kynar 761 polymer. The results are reproduced in Table 5:

TABLE 5 PVDF/ Tensile Nanotubes HFP Nanotubes Thickness Voltage CurrentResistivity Strength Batch # (g) (g) (%) (inch) (v) (amp) (ohm-cm) (psi)1 0.024 20 0.12 2 0.12 20 0.60 0.0095 0.4250 0.001 46.4563 2793 0.00950.2750 0.001 30.0599 0.0075 0.4150 0.001 35.8130 0.0080 0.6000 0.00155.2298 0.0085 0.5200 0.001 50.8574 0.0110 0.3500 0.001 44.2989 3 0.2220 1.09 0.0055 0.1025 0.001 6.4866 3296 0.0060 0.0700 0.001 4.83260.0090 0.0700 0.001 7.2489 4 0.64 20 3.10 0.0090 0.0075 0.001 0.77673343 0.0022 0.0380 0.001 0.9619 0.0110 0.0065 0.001 0.8227 5 1.08 205.12 0.0102 0.0048 0.001 0.5633 3206 0.0082 0.0052 0.001 0.4906 0.00500.0110 0.001 0.6328 0.0060 0.0095 0.001 0.6559 0.0080 0.0067 0.0010.6149 0.0100 0.0055 0.001 0.6328 6 2 20 9.09 0.0060 0.0036 0.001 0.24583695 0.0060 0.0034 0.001 0.2347 0.0035 0.0072 0.001 0.2908 0.0095 0.00240.001 0.2580 0.0060 0.0035 0.001 0.2444 7 3 20 13.04 0.0100 0.0012 0.0010.1323 0.0045 0.0017 0.001 0.0880 0.0055 0.0022 0.001 0.1392 0.00320.0025 0.001 0.0920 0.0080 0.0014 0.001 0.1289 8 1.52 10.1 13.08 0.00600.0015 0.001 0.1015 3165 0.0095 0.0008 0.001 0.0874 0.0100 0.0009 0.0010.1070 0.0050 0.0021 0.001 0.1208 9 1.54 10.04 13.30 0.0055 0.0154 0.0100.0975 0.0060 0.0124 0.010 0.0856 0.0045 0.0186 0.010 0.0963 0.00500.0125 0.010 0.0721 0.0050 0.0187 0.010 0.1076 0.0060 0.0125 0.0100.0863 10 1.95 10 16.32 0.0060 0.0012 0.001 0.0828 11 2.54 10.18 19.970.0065 0.0177 0.010 0.1322 0.0080 0.0120 0.010 0.1105 12 2.6 10.04 20.570.0035 0.0026 0.001 0.1047

The PVDF/HFP copolymer has lower crystallinity than PVDF polymer. Theresults of Example V showed that as little as 0.6% nanotube loadingresulted in a bulk resistivity as low as 30 ohm-cm. The bulk resistivityof the PVDF/HFP composite continued to drop as the nanotube loading wasincreased to approximately 13%. The bulk resistivity dropped below 1ohm-cm at 3.1% nanotube loading and the lowest reported bulk resistivityobserved was 0.072 ohm-cm at 13.3% nanotube loading, which is within therange of bulk resistivity for a pure CC nanotube mat. However, noimprovement in bulk resistivity was observed for PVDF/HFP compositeswith more than 13.3% nanotube loading. FIG. 1 illustrates the steepnessof the drop in bulk resistivity up to 3% nanotube loading and then therather linear decrease in bulk resistivity above 3% nanotube loading. Aninset plot within the graph of FIG. 1 was provided to better displaythis linear decrease in resistivity between 3 and 13% nanotube loading.

PVDF/HFP composites with nanotube loadings up to approximately 3%appeared to have lower bulk resistivities than those of PVDF compositeswith the same nanotube loadings. Thus, at low nanotube loading, theconductivity of a PVDF composite may be improved by using a PVDF/HFPcopolymer, or a lower grade PVDF with less crystallinity, instead of apure PVDF polymer. However, it was also observed that the tensilestrength of this PVDF/HFP composite is lower and thus the selection ofthe copolymer composite or the polymer composite will depend on theproperties required in the final application.

Conversely, the bulk resistivity of the PVDF composite is lower thanthat of the PVDF/HFP composite at higher nanotube loading (i.e., ˜20% orgreater). It was further observed that PVDF/HFP composites with over 16%nanotube loading had rough surfaces and holes, and were difficult tomold since they broke easily.

Example VI

A different grade of PVDF/HFP copolymer (Kynar 2801) was obtained fromElf Atochem and the procedure of Example I was repeated. Kynar 2801 isalso known as Kynar-Flex. The results are reproduced in Table 6:

TABLE 6 Kynar- Nanotubes Flex Voltage Current Thickness ResistivityBatch # (g) (g) Nanotubes (%) (v) (amps) (inch) (ohm-cm) 1 0.62 20 3.01% 0.0128 0.001 0.0075 1.1046 0.0075 0.001 0.0105 0.9061 2 1.06 20 5.03% 0.0093 0.001 0.0050 0.5350 0.0064 0.001 0.0075 0.5549 3 1.5 15 9.09% 0.0035 0.001 0.0075 0.3020 0.0019 0.001 0.0104 0.2274 0.00210.001 0.0090 0.2175 0.0010 0.001 0.0200 0.2186 4 1.51 10 13.12% 0.01050.010 0.0100 0.1208 0.0245 0.010 0.0040 0.1128 0.0115 0.010 0.01100.1456 5 1.54 10.14 13.18% 0.0033 0.001 0.0040 0.1519 0.0025 0.0010.0050 0.1438 0.0125 0.010 0.0100 0.1438 6 1.5 6 20.00% 0.0006 0.0010.0080 0.0506 0.0058 0.010 0.0078 0.0521 0.0004 0.001 0.0120 0.05250.0006 0.001 0.0080 0.0552

Unlike the PVDF/HFP composite of Example V, the Kynar 2801 copolymercomposite exhibited lower bulk resistivity above 13% nanotube loading.At 20% nanotube loading, the Kynar 2801 composite had bulk resistivityvalues of about 0.05 ohm-cm, which is within the range of a pure CCnanotube mat.

Example VII

PVDF composites were prepared by melt compounding PVDF (Kynar 761) andHyperion CC nanotubes and/or graphite (Lonza KS-75) in the mixing headof a Brabender mixer at 100 RPM for approximately five minutes at thetemperature specified. Each of the mixtures were prepared by sequentialaddition of the compounds in the following order: PVDF, nanotubes, thengraphite, unless the mixtures were premixed as indicated by an asterisk(*). Once compounded, flat sheets were prepared by pressing small piecesof the composite between thin, chromed plates at approximately 240° C.with the thin plates being cooled to room temperature in one to twominutes. Resistivity was measured with a linear four probe head. Theresults are reported below in Table 7:

TABLE 7 PVDF Nanotubes Graphite Nanotubes Temp Resistivity Batch # (g)(g) (g) (%) (° C.) (ohm-cm) 1 40 10 — 20 210   0.148* 2 42.5 7.5 — 15215   0.171 3 51 9 — 15 230   0.117 — 245   0.126 4 49.5 10.5 — 17.5 240  0.105 5 48 12 — 20 245   0.092 6 51 9 — 12.5 250   0.156 7 58.2 1.8 —3 240  12.1 —   1.93* 8 45.5 9 14 13.1 240   0.064 9 44.8 8.2 9 13.2 240  0.075 10 60 10 5 13.3 240   0.109 11 58 10 7.5 13.2 240   0.091 12 5610 10 13.1 240   0.077   0.055* 13 52 10 12.5 13.4 240   0.068 14 63.9 011.4 0 240 1000* 15 56 0 21.25 0 240  300*

The results show that very conductive composites can be formed by meltcompounding. Premixed materials appear to yield composites with lowerbulk resistivity than composites formed by sequential addition.

As shown in Batches 8-12, it was discovered that increasing the graphiteconcentration in PVDF composites at a given nanotube loading increasesthe conductivity of the composite. FIG. 2, which plots the resistivityfor Batch Nos. 8-12, illustrates the decrease in bulk resistivity as afunction of the increase in graphite concentration in the PVDF compositewith 13% nanotube loading.

Example VIII

Resistivity tests were performed on several polymer composites atseveral nanotube loadings. The composites were made using the solutionprocedure of Example I or the melt compounding procedure of Example VII.The following polymers were used:

PVDF-Sol (PVDF/nanotube composite made from solution);

Kynar-Flex (PVDF/HFP nanotube composite made from solution);

PVDF-Melt (PVDF/nanotube composite made by melt compounding);

PPS (poly(paraphenylene sulfide)/nanotube compound made by meltcompounding);

EVA (poly(co-ethylene-vinyl acetate)/nanotube compound made by meltcompounding);

PS (polystyrene/nanotube compound made by melt compounding); and

PE (polyethylene/nanotube compound made by melt compounding).

The results of Example VIII are shown below in Table 8:

TABLE 8 Nanotube Nanotube (weight (volume Resistivity fraction)fraction) Polymer (ohm-cm) 0.02 0.01 PPS 19.00 0.03 0.03 Kynar-Flex 1.100.91 0.04 0.03 PPS 211.00 0.05 0.03 PPS 3.12 0.05 Kynar-Flex 0.55 0.540.05 PVDF-Sol 0.42 0.07 0.06 PVDF-Sol 0.28 0.05 PPS 1.97 0.09 0.08Kynar-Flex 0.30 0.23 0.22 0.22 PVDF-Sol 0.23 0.11 0.10 PVDF-Sol 0.130.13 0.11 PVDF-Melt 0.16 0.12 Kynar-Flex 0.15 0.15 0.14 0.14 0.12 0.110.12 PVDF-Sol 0.09 0.15 0.11 PPS 0.25 0.25 0.14 PVDF-Melt 0.14 0.13 0.120.18 0.16 PVDF-Melt 0.10 0.20 0.09 EVA 0.41 0.18 PVDF-Melt 0.09 0.18Kynar-Flex 0.06 0.05 0.05 0.05 0.18 PVDF-Sol 0.04 0.09 PE 0.65 0.25 0.13PS 0.20 0.23 PVDF-Sol 0.05 0.26 0.12 PE 0.34 0.12 EVA 0.24 0.27 0.15 EVA0.25 0.13 EVA 0.23 0.14 PS 0.11 0.28 0.13 PE 0.29 0.29 0.14 PE 0.47 0.300.16 PS 0.15 0.14 EVA 0.13 0.33 0.16 EVA 0.20

The nanotube weight fraction was calculated by dividing the nanotubeweight by the composite weight. The nanotube volume fraction wascalculated by dividing the volume of the nanotubes by the volume of thecomposite. These volumes were calculated by dividing each of thenanotube and polymer weights by their respective densities (the volumeof the composite is the sum of the nanotube and polymer volumes).

The results of Example VIII showed that the resistivities of the PVDFand PVDF/HFP composites are orders of magnitude lower than theresistivities of even the best conductive polymers at any given nanotubeloading level. For example, at 5% nanotube loading, the PVDF andPVDF/HFP composites had bulk resistivity values ranging from 0.42 to0.55 ohm-cm, while the bulk resistivity of the PPS composite was 3.12ohm-cm. At 20% nanotube loading, The PVDF and PVDF/HFP composites hadbulk resistivity values between 0.04-0.09 ohm-cm, which is within therange of a pure CC nanotube mat. No other polymer composite at 20%nanotube loading or at any higher nanotube loading level had a bulkresistivity value within the range of that of a pure CC nanotube mat.These differences are significant for applications where high electricalconductivity is crucial.

FIG. 3 sets forth a logarithmic plot of nanotube weight vs. resistivity.A line is drawn which unequivocally distinguishes the resistivity of thePVDF composites from all other polymer composites, illustrating clearlythat PVDF composites are superior to other polymer composites inelectrical conductivity.

FIG. 4 sets forth a plot of nanotube weight vs. conductivity. As FIG. 4confirms, PVDF composites have clearly superior conductivity than otherpolymer composites known in the art. Additionally, FIG. 4 further showsthat, unlike other polymer composites known in the art, conductivity forPVDF composites increase exponentially as the nanotube loading isincreased to approximately 20%. PVDF composites also obtained theconductivity of a pure CC nanotube mat (i.e., 12.5-50/ohm-cm).Composites made from other polymers were unable to reach theconductivity range of a pure CC nanotube mat, even as the nanotubeloading was increased beyond 30%.

Example IX

Multilayered structure comprising a first layer of a PVDF composite, asecond layer of a thermoplastic or thermoplastic blend/composite, and anoptional third adhesive layer between the first and second layers. Thesecond layer can be a nylon-6 (6,6, 11, or 12), a nylon-clay compositeknown for excellent barrier and high heat distortion properties, or anylon blend. The adhesive layer can be a PVDF-nylon blend withrelatively lower viscosity. This layered structure can be fabricated ina sheet form, or into a container of any shape and size, or into atubing/pipe. The inner layer for the container and tubing forms ispreferably a PVDF composite layer. Since the PVDF polymer is known forits resistance to heat and hydrocarbons, and the nylon material, inparticular, nylon-clay composite is known for its high heat distortiontemperature, excellent barrier properties and good mechanicalproperties, containers and tubing formed from this multilayeredstructure can be used for safe storage and transport wide-range ofhydrocarbons.

A multilayered tubing may be prepared using the following materials:PVDF composite of Kynar 761 and 13% loading nanotube, clay-nylon-6composite and a PVDF (Kynar 741)-Nylon-6(30%) blend. The preparation ofa three-layer tubing was carried out in a coextrusion system equippedwith three extruders. The inner diameter of the tubing and thickness ofeach layer are: inner diameter: 20 mm; thickness of inner PVDF compositelayer: 0.6 mm; thickness of adhesive PVDF-nylon blend layer: 0.2 mm;thickness of outer clay-nylon composite layer: 1 mm.

Example X

A PVDF emulsion, Kynar 720, was obtained from Elf Atochem. Hyperion CCcarbon nanotubes were added and dispersed into the Kynar 720 emulsionusing a high shear blender (i.e., Waring blender). The water/liquid sentin this mixture was removed by evaporation and thin specimens of thePVDF composition with dimensions of approximately 0.5′×3″×0.01′ wereprepared by hot press. The resistivities were measured and reportedbelow:

Nanotubes Resistivity (%) (Ohm-cm) 3 3.11 13 0.14 20 0.063

The results of Example X confirm that PVDF composites with low bulkresistivity may be prepared using PVDF emulsions. At 20% nanotubeloading, the PVDF composite prepared using this method had a bulkresistivity within the range of a pure CC nanotube mat.

We claim:
 1. A method for preparing an electrically conductive compositecomprising the steps of: (a) mixing carbon nanotubes with a polymeremulsion, said emulsion comprising a liquid and a polymer selected fromthe group consisting of polyvinylidene fluoride and copolymer ofvinylidene fluoride and another monomer; and (b) removing said liquid toform a composite comprising said nanotubes and said polymer, whereinsaid nanotubes have a diameter less than about 100 nanometers.
 2. Themethod of claim 1, wherein the liquid is water.
 3. The method of claim1, wherein said removing step is performed by evaporating said liquid.4. The method of claim 1, wherein said mixing step is performed with ahigh shear blender.
 5. The method of claim 1, wherein said mixing stepis performed with a Waring blender.
 6. The method of claim 1, whereinsaid monomer is selected from the group consisting ofhexafluoropropylene, polystyrene, polypropylene,chlorotrifluoroethylene, tetrafluoroethylene, terpolymers or olefins.